Identify the least stable ion amongst the following 

Atomic number of Be is 4 and its electronic configuration is 1s² 2s². Be already has a filled stable configuration and if one additional electron is added then it disturbs the stable configuration of Be. 

$\mathrm{Be}+1{\mathrm{e}}^{-}\to {\mathrm{Be}}^{-}$

Therefore Be^- is highly unstable among the given options.

Atomic number of Li is 3 and its electronic configuration is 1s² 2s¹. If one electron is added to Li, then it forms Li^-.

$\mathrm{Li}+1{\mathrm{e}}^{-}\to {\mathrm{Li}}^{-}$

Electronic configuration of Li^- is 1s² 2s². After addition of one electron, 2s orbital of Li gets filled and Li achieves a stable configuration. Therefore, Li^- is more stable than Be^-.

Atomic number of B is 5 and its configuration is 1s² 2s² 2p¹. If one electron is added, B forms B^-.

$\mathrm{B}+1{\mathrm{e}}^{-}\to {\mathrm{B}}^{-}$

Electronic configuration of B^- is 1s² 2s² 2p². Boron does not have a stable configuration, so the addition of an extra electron won't disturb the stable configuration of B as happens in the case of Be.

 Atomic number of C is 6 and its electronic configuration is 1s² 2s² 2p². C forms C^-, when one electron is added to C.

$\mathrm{C}+1{\mathrm{e}}^{-}\to {\mathrm{C}}^{-}$

Electronic configuration of C^- is 1s² 2s² 2p³. The electronic configuration of C^- is more stable than C because it is half-filled. Therefore, addition of one electron in C provides stability to C.